The principle of the absorption cycle of such machines is the following: the working fluid, which is initially in the gaseous phase, undergoes condensation, expansion, and vaporization steps, then an absorption step in a solvent fluid. The resulting solution is pressurized again. An increase in the temperature regenerates the working fluid in the gaseous phase in a boiler. The solvent is then sent to the absorption step.
The working fluid-solvent fluid pairs which are most frequently used are the NH.sub.3 --H.sub.2 O and H.sub.2 O--LiBr pairs. However, these pairs exhibit major drawbacks which prevent their use in certain areas, particularly in individualized or collective heating operations. In fact, the NH.sub.3 --H.sub.2 O pair, which is interesting from a thermodynamic point of view, has restricted use because NH.sub.3 is toxic. The H.sub.2 O--LiBr pair cannot be used at the low temperatures required in the evaporator when used in a heat pump to heat individual dwellings or group dwellings because of the crystallization of the water.
In an absorption cycle, it is important that the working fluid-solvent fluid pair exhibits good thermodynamic properties, that a satisfactory solubility of the working fluid in the solvent exists, and that it is not toxic and does not crystallize.
In addition, it is preferred that the solvent have a normal boiling point which is preferably higher than 100.degree. C. to facilitate separation in the boiler.
For this purpose, the use of chlorofluorinated hydrocarbons as working fluids with heavy compounds as solvents has already been proposed (Revue Generale de Thermique -Nos. 236-237 (August-September, 1981).
The use of these working fluid-solvent fluid pairs in an absorption cycle is certainly interesting, particularly because the working fluid has a very high solubility in the solvent. However, such pairs decompose at high temperatures when they are in contact with a metal compound, while each component of the pair is thermally stable under the same conditions. However, it is crucial for a heating installation with an absorption cycle to have a working fluid-solvent system which is stable for a period equivalent to 10 years of operation. This is also of crucial important for air conditioning and refrigeration installations.
Consequently, the proposal has been made to inhibit the decomposition reaction by introducing, in these working fluid-solvent fluid pairs, suitable and effective stabilizing additives as described in U.S. Pat. No. 4,612,133.
It has also been suggested to use mixtures of chlorofluorinated hydrocarbons and chlorofluorinated compounds, without the addition of a stabilizer, as fluid pairs for absorption heat pumps, as described in French patent application No. 2,575,174 and U.S. Pat. No. 4,647,391.
It is also possible to use fluorinated compounds which do not belong to the group of chlorofluorinated hydrocarbons having 1 or 2 carbon atoms. An example is European Patent No. 143,509, which proposes the use, as a working fluid, of acyclic, linear, branched, or cyclic hydrocarbons and fluorinated ethers containing from 3 to 5 carbon atoms, with a general formula which exclusively contains carbon, hydrogen, fluorine, and oxygen, for the ether used.
When fluorinated ethers are used as working fluids, the solvent is tetraglyme or, more generally, any compound with proton-acceptor properties.
The use of fluorinated ethers as working fluids is particularly interesting because these compounds are not toxic and are therefore already used in anesthesia. In addition, these compounds are very soluble in most of the solvents, especially when they contain chlorine atoms. On the other hand, however, the presence of this element results in a lowering of the stability of the working fluid-solvent pair in the presence of metals at temperatures usually reached in the boiler of absorption apparatus.
In fact, it is well known that a chlorofluorinated hydrocarbon of the C.sub.n H.sub.2n+2-x-y F.sub.x Cl.sub.y type, when heated in the presence of iron, aluminum, or copper, and a hydrogen donor compound, undergoes a transformation which can sometimes be quantitative, with substitution of the chlorine atoms by hydrogen atoms from the solvent. Thus, it has been shown in U.S. Pat. No. 4,612,133 that 1-chloro-2,2,2-trifluoroethane, under these conditions, leads to trifluoroethane according to the following reaction scheme: EQU CF.sub.3 --CH.sub.2 Cl+Fe.fwdarw.CF.sub.3 --CH.sub.2.sup.. +FeCl.sup.. EQU CF.sub.3 --CH.sub.2.sup.. +R--H(solvent).fwdarw.CF.sub.3 --CH.sub.3 +R.
In chlorofluorinated hydrocarbons which contain at least 2 carbon atoms, olefin is formed: EQU CF.sub.3 --CH.sub.2.sup.. +FeCl.sup.. .fwdarw.CF.sub.2 =CH.sub.2 +FeClF
In chlorofluorinated ethers, this instability is amplified by the presence of the ether function. Thus, 1-methoxy-1,1,2-trifluoro-2-chloroethane (CH.sub.3 --O--CF.sub.2 --CHFCl) is completely decomposed after 100 hours at 180.degree. C. regardless of the solvent used. The same holds for 1-difluoromethoxy-1-chloro-2,2,2-trifluoroethane (isoflurane: CF.sub.2 H--O--CHCl--CF.sub.3), with the principal decomposition products resulting from the transfer of the chlorine atom onto fragments of the solvent molecules being: CH.sub.3 Cl, C.sub.4 H.sub.9 Cl, etc.
The preceding references are hereby incorporated by reference.